1. Field of the Invention
The present invention generally relates to a magnetic tape recording and/or reproducing apparatus generally referred to as a video tape recorder and, more particularly, to a head-carrier drum control apparatus, used in the video tape recorder, of a type comprising at least one rotary magnetic recording and/or reproducing head supported for displacement in a direction generally widthwise of a length of magnetic recording medium to accomplish proper tracking for a high quality video reproduction.
2. Description of the Prior Art
It is well known that most video tape recorders of helical scan type now available in the market employ a generally cylindrical head-carrier drum assembly comprising rotary and stationary drum halves, the rotary drum half carrying a magnetic recording and/or reproducing head assembly for rotation together therewith. An example of the prior art head-carrier drum assembly is reproduced in FIG. 7 of the accompanying drawings in a schematic representation and will now be discussed with reference thereto.
Referring now to FIG. 7, the head-carrier drum assembly shown therein comprises a motor-coupled drive shaft 1, a stationary or lower drum half 2 fixedly mounted on a support framework (not shown) and carrying a pair of spaced apart bearings 6 through which the drive shaft 1 extends coaxially for rotation independently of the lower drum half 2, and a rotary or upper drum half 3 mounted on the drive shaft 1 through a support stock 9, rigid with the drive shaft 1, for rotation together therewith and in coaxial relation with the lower drum half 2, the lower drum half 2 having an outer diameter exactly equal to the outer diameter of the upper drum half 3. The upper drum half 3 carries a plurality of, for example, two, magnetic recording and/or reproducing heads 5a and 5b for rotation together therewith spaced 180.degree. from each other about the axis of rotation of the upper drum half 3 that is defined by the drive shaft 1. These magnetic transducing heads 5a and 5b are secured from below to the upper drum half 3 by means of respective fixture plates 4 with tips of said transducing heads 5a and 5b confronting a circumferentially extending clearance between the lower and upper drum halves 2 and 3. Within the head-carrier drum assembly, sector-shaped transformers 7 are supported by the support stock 9 for rotation together with the drive shaft 1 or the upper drum half 3, and correspondingly sector-shaped transformers 8 cooperable with the upper transformers 7 are rigidly mounted on the lower drum half 2. A length of magnetic tape generally identified by 13 and reeled at opposite ends to supply and take-up reels (not shown) is, during use, wrapped helically around the head-carrier drum assembly while extending at a predetermined angle relative to the longitudinal axis of the head-carrier drum assembly.
In this illustrated prior art structure, the magnetic transducing heads 5a and 5b are positioned at respective predetermined locations spaced 180.degree. from each other about the drive shaft 1 and retained by the upper drum half 3 with their tips protruding a slight distance outwards from the outer peripheral surface of the upper drum half 3. The magnetic transducing heads 5a and 5b are electrically connected with the upper transformers 7 through connectors 10a and 10b, then through printed wiring boards 11a and 11b and finally through connectors 12a and 12b, respectively. During a fixed speed rotation of the upper drum half 3 driven by the drive shaft 1 while the length of magnetic tape 13 is transported at a predetermined speed in one direction from the supply reel towards the take-up reel, the magnetic transducing heads 5a and 5b while alternately switched into operation scan that portion of the length of magnetic tape 13 which is then wrapped around the head-carrier drum assembly, thereby to either record or reproduce one or both of video information and audio information.
As described above, each upper transformer 7 is fixed to the support stock 9 rotatable together with the drive shaft 1 and is spaced a slight distance from and confronts the associated lower transformer 8 which is electrically connected with a signal processor (not shown in FIG. 7). During operation, the sector-shaped upper and lower transformers 7 and 8 are magnetically coupled to transmit a signal therebetween.
Referring still to FIG. 7, the upper drum half 3 has a magnet 75 mounted atop the upper drum half 3 adjacent an outer peripheral portion thereof, which magnet 75 is cooperable with a magnetic position sensor 76 supported immediately above the path of movement of the magnet 75 for detecting magnetic lines of force emanating from the magnet 75 thereby to provide a signal indicative of the position of the upper drum half 3 relative to the lower drum half 2.
The prior art head-carrier drum assembly of the construction described above operates in the following manner.
During information recording or reproduction, the magnetic transducing heads 5a and 5b successively scan the length of magnetic tape 13 in a direction slantwise with respect to the longitudinal axis of the length of magnetic tape 13 while depicting successive traces which are parallel to each other as shown in FIG. 8. It is to be noted that the traces of movement of the magnetic transducing heads relative to the movement of the length of magnetic tape 13 are generally referred to as the recorded tracks on the length of magnetic tape 13 when the latter is used for reproduction of previously recorded information and referred to as the recording tracks on the length of magnetic tape 13 when the latter is used for recording information thereon.
In FIG. 8, reference numeral 13a represents a trace of movement of the length of magnetic tape 13; reference character V1 represents the speed of movement of the length of magnetic tape 13 during information recording or reproduction; reference numeral 5A represents a trace of movement of any one of the magnetic transducing heads 5a and 5b relative to the length of magnetic tape 13; and reference character V0 represents the direction of movement of any one of the magnetic transducing heads 5a and 5b. As shown, the trace 13a of movement of the length of magnetic tape 13 and the trace 5A of movement of any one of the magnetic transducing heads 5a and 5b generally intersect with each other at a predetermined angle.
Accordingly, tracks depicted by any one of the magnetic transducing heads 5a and 5b on the length of magnetic tape 13 during the information recording or normal information reproduction are such as generally identified by A in FIG. 8(a) and the space P between the neighboring traces A represents a track pitch. However, when the speed of movement of the length of magnetic tape 13 is increased during information reproduction from the value V1 to a higher value V2 such as occurring when an operator selects a high speed search mode to skip a video program being reproduced and to locate the succeeding video program, or during a high speed picture reproduction or quick-motion picture reproduction, the traces of movement of any one of the magnetic transducing heads 5a and 5b depicted on the length of magnetic tape 13 will be such as shown by B in FIG. 8(b).
Thus, it is well known that, while the peripheral velocity of any one of the transducing heads 5a and 5b is fixed at a predetermined value, the length of magnetic tape 13 is transported at a speed different from that during the normal information reproduction such as occurring during any one of the search mode, a quick-motion picture reproduction, a slow-motion picture reproduction or a still or frozen picture reproduction. During such operation, if any one of the magnetic transducing heads 5a and 5b fail to properly follow the recorded tracks on the length of magnetic tape 13, signals picked up by the magnetic transducing heads 5a and 5b are lowered in level. The consequence is the reproduction of video and audio information of reduced quality and also the appearance of noise bars in the video display.
In order to substantially eliminate the above discussed problems, attempts have hitherto been made to permit the magnetic transducing heads to be displaceable according to the change of the speed of movement of the length of magnetic tape, or tape speed for short, relative to that during the normal picture reproduction or, more precisely, relative to the tape speed at which the length of magnetic tape had been transported during the information recording. For example, the Japanese Patent Publication examined No. 56-50329, published Nov. 28, 1981, discloses the use of two head position control elements each capable of displacing in dependence on the magnitude of an electric signal applied thereto and having a respective movable member. The magnetic transducing heads are secured to the rotary drum half through the respective movable members of the head position control elements so that, when an electric signal appropriate to a particular operating mode such as a high speed search mode or a slow-motion picture reproduction is applied to the head position control elements, the magnetic transducing heads 5a and 5b can be displaced in a direction parallel to the axis of rotation of the rotary drum half to allow the magnetic transducing heads to properly follow the discrete parallel tracks on the length of magnetic tape.
U.S. Pat. No. 4,318,142, patented Mar. 2, 1982, to Raymond F. Ravizza, discloses the automatically compensated movable head servo circuit wherein, in order to cause the magnetic transducing head to accurately follow the tracks during the picture reproduction and, at the completion of the picture reproduction from the tracks, to properly position the magnetic transducing head in position to either reproduce the next adjacent successive track, reproduce the same track or reproduce another track so that the appropriate special motion effect can be achieved, a technique is employed to apply a small oscillatory motion to the magnetic transducing head to cause it to vibrate laterally of the track, to examine the resulting modulation of the reproduced signal's envelope to generate a tracking error correction signal and to apply the error correction signal to the magnetic transducing head.
U.S. Pat. No. 4,451,859, patented May 29, 1984, to Stan L. Noel discloses a magnetic head position control system wherein a constant control potential is applied to a positionable magnetic transducing head during information recording to hold the magnetic transducing head fixed. The position of the magnetic transducing head relative to the tracks on the length of magnetic tape is controlled during picture reproduction in dependence on signals reproduced from the tracks on the length of magnetic tape.
The above discussed prior art systems employ a drive unit including a head position control element necessitated either to displace the magnetic transducing head such as in the first mentioned publication or to cause the magnetic transducing head to finely oscillate such as in the second and last mentioned publication, which drive unit referred to above has a mechanical resonant frequency peculiar to such drive unit. Since in any one of the above discussed prior art systems the drive unit is driven at a frequency associated with the cycle of rotation of the head-carrier drum, that is, the rotary drum half, with no particular care paid to the mechanical resonant frequency peculiar to the drive unit, the drive unit tends to undergo mechanical resonance at a high frequency region and, therefore, the trace of movement of the magnetic transducing head during picture reproduction fluctuates, as shown by the broken line in FIG. 9(b), relative to the trace of movement of the magnetic transducing head depicted during information recording as shown by the solid line in FIG. 9(b). The consequence is that proper tracking cannot be maintained and, therefore, the envelope of the reproduced signal oscillates, as shown in FIG. 9(c), and is accompanied by the generation of noises at each neck region (region of small amplitude) of the envelope which thereby causes a reduced signal-to-noise ratio.
Also, when the drive unit is driven by a voltage of a frequency including a fundamental drive frequency thereof, a loss of the resistance of the head position control elements multiplied by the second power of the control current applied to the head position control elements of the drive unit, accompanied by the self-heating of the head position control elements. Once this self-heating occurs, the ambient temperature changes to such an extent as to result in change in resistance of the head position control elements and, also, change in contact resistance between contact elements and associated electrodes, both used to supply the control current there through to the drive unit. Once the resistance of the head position control elements and/or the contact resistance between the contact elements and the associated electrodes vary as discussed above, the supply of the control current to the drive unit is no longer stabilized, resulting in improper head-to-tape tracking accompanied by the generation of noise bars in the video display.